He will display, they predict, the same traits—attractive and not-so--that have taken him, at age 56, to the pinnacle of the banking industry.

His lightning career has taken him from American Express to Citigroup (which he formed with his then-mentor, Sandy Weil, in 1998), to Banc One, where he became CEO in 2000. In 2004, when Banc One was purchased by JPMorgan, he became president of the combined company, then later CEO. On his watch, JPMorgan has grown to be the biggest U.S. bank in terms of market capitalization and assets under management.

In his prepared remarks, made available before the hearing, Dimon defended JPMorgan's performance, even while admitting that the bank felt "terrible" for having lost some of its shareholders' money.

As for what Dimon may say in response to questions, one woman who advised him early in his career predicts, "He will speak from the heart." He'll be candid, she says, and forthright. He won't mince words. "He's not coachable, not in the sense that he will listen to experts and then do what they tell him to do. He's his own man. I know that from my direct experience with him."

Like other former Dimon colleagues interviewed for this story, she requested to be nameless.

Says Patricia Crisafulli, one of Dimon's biographers, "He's known as a straight talker--not just forthright but honest." Her book, "The House of Dimon: How JPMorgan's Jamie Dimon Rose to the Top of the Financial World" depicts him as nothing if not direct. With Jamie, Crisafulli writes, what you see is what you get.

That sometimes can include cocksureness, petulance and temper.

"I think very highly of him," says a Dimon colleague from the 1990s. "There's no bulls**t about him. People respect that. But he's blunt to a fault--rough around the edges. Sometimes, he's too smart for his own good, finishing sentences for you, even when what he said wasn't' what you had in mind. He jumps to conclusions."

He jumped to a false one on April 13, when, in a conference call with JPMorgan analysts, he peremptorily dismissed worries over trades made by JPMorgan's London office as "a tempest in a teapot." That tempest, the Wall St. Journal reports, now stands to lash the bank with losses of $5 billion, maybe more.

Though Dimon since has castigated himself publicly for having misjudged the situation, shareholders, analysts and politicians are looking for a scalp.

Dimon insists that the full magnitude of the London trading risk was not known to him, and that he remained ignorant of key details until it was too late to prevent the loss. That surprises his '90s colleague, who thinks of Dimon as the ultimate detail guy: "What surprises me most in this whole debacle is that he really was a detail guy. Nothing slipped by him. My guess is he may have delegated too much and not have known. If he had, I don't see how it could have reached this level of loss."

Says his former advisor, "He has a maniacal desire to have his hands on all the details. My guess is, he's truly beating himself up for what went wrong."

Crisafulli in her book describes Dimon as someone unafraid to get bad news—in fact, as a leader who encouraged subordinates to give critical feedback. Dimon, she writes, wanted to know about any practice or issue that threatened to detract from the companies he managed.

All the sources interviewed for this story dismissed the suggestion that Dimon has enjoyed a charmed career, noting that he's taken his share of lumps. "I wouldn't say it's been charmed," says his 1990s co-worker. "His relationship with Sandy Weil was not smooth sailing. Very soon after Jamie began to rise, Sandy viewed him as a threat."

Dimon in 1998 was fired by his former mentor. "Fired summarily," corrects his former advisor. "He went through a very difficult time after that. But he used to tell me, you're not defined by how you behave on your way up but on how you behave on your way down. You're defined by how you handle failure."

As for how Dimon will now address JPMorgan's problems, biographer Crisafulli points to how Dimon dealt with a previous financial crisis at Banc One: "He recognized the problems and addressed them; he got to the bottom of it, and he brought about a positive result."

"His instincts are good," says his one-time advisor. Corny as it may sound, she says, Dimon has "an intense desire to do the right thing for his country, for his company and for the people who work for it--in that order. At the end of the day, he will do what he believes is the right thing to do."